Does partial tracing make us see things that do not occur?

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Discussion Overview

The discussion revolves around the concept of partial tracing in quantum mechanics and its implications for our understanding of observable phenomena. Participants explore whether partial tracing leads to the perception of events or entities that do not actually occur, drawing analogies and questioning the nature of measurement and entropy in quantum systems.

Discussion Character

  • Exploratory
  • Debate/contested
  • Conceptual clarification

Main Points Raised

  • Some participants propose that partial tracing maps phenomena from a larger Hilbert space to a smaller one, suggesting that this process might create perceptions similar to "intersection points" in a lower-dimensional view.
  • Others argue that the emission of a photon can be considered an event, questioning the nature of events in quantum mechanics.
  • A participant references historical experiments involving charged particles and their trajectories, indicating that these observations do not contradict quantum theory.
  • One participant challenges the metaphor of dimensionality in relation to partial tracing, suggesting it is more akin to marginal probability computation.
  • Another point raised is that partial tracing can lead to the calculation of entropy, which some participants discuss in the context of the universe's low initial entropy and its implications for observers.
  • There is a suggestion that partial tracing might obscure certain occurrences rather than reveal them, presenting a counterpoint to the initial premise of the discussion.
  • Concerns are raised about the objectivity of entropy, with references to Rovelli's doubts regarding our observational perspective and its influence on perceived entropy.
  • Participants mention open problems in quantum mechanics, including the measurement problem and ontology problem.

Areas of Agreement / Disagreement

The discussion features multiple competing views regarding the implications of partial tracing, the nature of observable events, and the objectivity of entropy. There is no consensus on whether partial tracing leads to the perception of non-occurring phenomena or obscures actual occurrences.

Contextual Notes

Participants express uncertainty about the implications of partial tracing, the definitions of events, and the nature of entropy, indicating that these concepts may depend on the observer's perspective and the specific context of measurement.

Heidi
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Hi Pfs
Partial tracing maps what occurs in a big Hilbert space toward a smaller one. We have to use it when degrees of freedom are physically unobservable or when we have only a coarse grained view of the environment. it is like in Flatland , where the two dimensional inhabitants has no access to the third dimension. non they only see the shadows of what is above. for them non parallel lines always have an intersection point even if this point does not exist above them. They see this point !
Are there also for us things that we see and measure which are like these intersection points ( due to partial tracing)?
 
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we know that particles do not follow trajectories but we see the pictures of charged particles in a magnetic field. Is it not the same for the notion of event? is the emition of a photon an event like those in our life?
 
I guess you mean the tracks charged particles make in a cathode ray tube showing "classical trajectories" under the influence of electric and/or magnetic fields and which were used in the early 1900ds to check Einstein's special theory of relativity (e.g., by Kaufmann) vs. other electron-theoretical models (most famously Abraham's).

This is of course no contradiction to quantum theory and well understood since 1929:

N. Mott, The Wave Mechanics of alpha-Ray Tracks,
Proceedings of the Royal Society of London. Series A 126, 79
(1929), https://doi.org/10.1098/rspa.1929.0205
 
Heidi said:
non they only see the shadows of what is above. for them non parallel lines always have an intersection point even if this point does not exist above them. They see this point !
I don't think that it's a good metaphor for partial tracing. Partial tracing is just a quantum analog of marginal probability computation.
https://en.wikipedia.org/wiki/Marginal_distribution
 
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Heidi said:
Are there also for us things that we see and measure which are like these intersection points ( due to partial tracing)?
An example of measurable quantity calculated with the help of partial tracing is entropy.
 
Heidi said:
Does partial tracing make us see things that do not occur?
It's actually the opposite, often partial tracing makes us not see things that occur.
 
Yes partial tracing can give the entropy. It shows that at the beginning the entropy of the universe was very very low. it is what we calculate and "see". but is it an objective thing?
Rovelli has doubts. maybe it looks like that because WE are now in a peculiar region or we are peculiar observers. this low entropy correspond to our coarse grained point of view.

what are for you the open problems in QM?
 
Heidi said:
Yes partial tracing can give the entropy. It shows that at the beginning the entropy of the universe was very very low. it is what we calculate and "see". but is it an objective thing?
Rovelli has doubts. maybe it looks like that because WE are now in a peculiar region or we are peculiar observers. this low entropy correspond to our coarse grained point of view.
Yes entropy is objective, and yes entropy depends on our coarse graining. I don't see a contradiction.

Heidi said:
what are for you the open problems in QM?
The measurement problem and (closely related) the ontology problem.
 
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